High temperature surface treatment apparatus

ABSTRACT

Apparatus for enabling a plasma gun to operate in a transferred arc mode when effecting a surface treatment of refractory materials that are not electrically conductive at room temperature. The apparatus includes a counter electrode that may be positioned adjacent the workpiece the use of which enables a portion of the refractory surface to be heated sufficiently for some ionization to take place to permit efficient arc working and enable a deep fused layer to be formed.

lnventors England [21] Appl. No 712,839 [22] Filed Mar. 13,1968 [45] Patented June 8, 1971 [73] Assignee [54] HIGH TEMPERATURE SURFACE TREATMENT APPARATUS 7 Claims, 6 Drawing Figs.

[52] U.S.Cl 219/121, 219/137 [51] lnt.Cl 823k 9/00 [50] Field of Search 219/121, 121 P, 137, 74, 75, 76, 543;117/93.1,105.2,123, 125

[5 6] References Cited UNITED STATES PATENTS 1,924,876 8/1933 Morgan 2l9/76X 2,194,611 3/1940 Paddock etal 117/125X 2,264,499 12/1941 Bair 117/125X 2.306,054 13/194. Guyerl. 219/121X 2.703767 3/1955 Young. 2l9/543X 3,020,182 2/1962 Daniels 117/125X 3,049,447 8/1962 Knapp. 117/123X 3,075,066 l/1963 Yenniet al. 117/93.1X 3,311,735 3/1967 Winzeler et a1. 219/121 2,111,872 3/1938 Rea l 219/121 2,306,054 12/1942 Guyer 219/121 2,874,265 2/1959 Reed et al 219/121 3,071,678 l/l963 Neely et al. 219/121 3,248,513 4/1966 Sunnen 219/121 3,358,114 12/1967 lnoue 219/121 Primary ExaminerJ. V. Truhe Assistant Examiner-C. L. Albritton Attorney-Townshend and Meserole ABSTRACT: Apparatus for enabling a plasma gun to operate in a transferred arc mode when effecting a surface treatment of refractory materials that are not electrically conductive at room temperature. The apparatus includes a counter electrode that may be positioned adjacent the workpiece the ufe of which enables a portion of the refractory surface to be heated sufficiently for some ionization to take place to permit efficient are working and enable a deep fused layer to be formed.

PATENTEU JUN sum 3 5 4 1 4 sum 2 or 2 HIGH TEMPERATURE SURFACE TREATMENT APPARATUS This invention relates to high temperature apparatus. It relates specifically to apparatus for the production of a constricted are plasma which may be used particularly for the treatment of surfaces.

In the production of constricted arc plasma the plasma generation apparatus may be arranged to operate in either of two modes, as a nontransferred are or a transferred arc mode. In the latter mode, a workpiece to be treated with a heated effluent from the apparatus is in circuit with the arc. The work piece is usually made a positive pole in the arc circuit which enhances considerably the heating effect on the workpiece.

The use of the transferred are system therefore requires the workpiece to be of electrically conductive material so that it can form part of the arc circuit and conduct the high currents produced in this type of are. If the workpiece is nonconductive then the conventional technique is to use a nontransferred arc system in which possibly a nozzle through which the plasmajet emerges from the generation apparatus acts as the electrode. With the nontransferred are system the amount of heat energy dissipated in the workpiece is much smaller than in the transferred are system.

The transferred are system therefore makes better use of the heating effects that can be obtained from the arc but this system has hitherto been restricted to use with workpieces that were electrically conductive. We have found that this system can be extended in its application to workpieces of certain materials that are nonconductive at ordinary temperatures and the system then provides an efficient way of treating these materials with the plasma jet.

According to one feature of the invention, high temperature apparatus for the generation of plasma by a transferred arc system comprises a counter electrode which may be connected to a plasma generator and which may be placed on or near to a workpiece adjacent to a point thereon which is required to be acted upon by a plasmajet from the generator. Preferably the counter electrode is of copper and it may include means for keeping the electrode cool. The cooling means may comprise channels through which a stream of cooling water can be passed during operation of the electrode.

The counter electrode may be positioned upon a support so that the electrode is a short distance from the point on the workpiece which is to be acted upon by the plasma stream. The use of such a support helps to reduce contamination of the material of the workpiece by the material of the electrode -and vice versa. The provision of a separate support for the electrode also enables movement of the workpiece to be effected without this movement affecting the position of the counterelectrode with respect to the plasma generator. Therefore the workpiece may be moved about as necessary so that a large area of the workpiece surface may be treated in sections by the plasma jet.

Where an extended surface of a workpiece is to be treated the counterelectrode may be in the form of an elongated plate and the plasma generator may be arranged to move repeatedly in a path parallel to this plate so that the whole surface of the workpiece will be treated stage by stage as a jet nozzle of the plasma generator is moved to and fro along the length of the counterelectrode.

This method has been found to produce a very high temperature region on the surface of the refractory material which is sufficiently high to evaporate and partially ionize the material of the surface. The kinetic energy of the arc impinging on the surface also helps to shift molten and partly molten material from the surface of the refractory and thus permits deep penetration. In this way it has been found possible to produce very deep deposits of fused surfaces and the properties of these surfaces may be modified by the incorporation of suitable additives in the surface.

The invention also envisages the production of laminated or nonhomogeneous surfaces in which the properties thereof may be made to vary sharply or smoothly as the depth of the surface is penetrated. By way of example the invention will be further described with reference to the accompanying drawings in which,

FIG. I is a perspective view of apparatus according to the invention arranged for are plasma treatment of a refractory brick,

FIG. 2 is a cross-sectional view taken along the line ll-ll on FIG. I looking in the direction shown,

FIG. 3 is a view partly cut away of apparatus for the treatment of a cylindrical refractory body,

FIG. 4 is a part cross-sectional view taken along the line IVIV on FIG. 3,

FIG. 5 is a refractory brick having a fused surface layer according to the invention, and

FIG. 6 is a refractory sleeve having a fused surface layer.

In a first example, the operation of using a plasmajet to seal the surface of a refractory brick was undertaken. For many years the surface sealing of ceramic bodies has been effected by applying a ceramic glaze to the usual porous surface of the ceramic and then firing the glaze to convert it to a glassy impermeable surface skin. A method of forming a ceramic glaze in situ on the surface of a ceramic was disclosed in out copending British Pat. application No. 49,882/65 (British Pat. Ser. No. 1,172,825). The present invention provides a method of forming a much thicker coating of glaze on the ceramic surface which will add to the structural strength of the ceramic and even enable two pieces of ceramic lying alongside one another to be joined along a common edge.

FIG. 1 shows a refractory brick l which is positioned between two sidewalls 2. The sidewalls 2 are of copper which is kept cool by the passage of water through cooling pipes 3. For the sake of clarity in the drawings the cooling pipes 3 are illustrated only in FIG. 2.

At one end of a short channel defined by the sidewalls 2, a counter electrode 4 rests on the top edges of the sidewalls but is electrically insulated therefrom. The sidewalls 2 are of such a height that the electrode is supported at a height of 3 millimeters above an upper surface of the refractory brick l. The counter electrode 4 is formed of a copper sheet bent to a boxlike section and carries water cooling pipes 5 for the removal of heat from the copper.

The counter electrode 4 also carries a terminal 6 by means of which the electrode may be connected to a plasma generator to enable transferred arc operation of the generator to take place. FIG. I shows a nozzle 7 of such a generator with a plasma jet 8 emerging from this nozzle. The other necessary parts of the plasma generator have been omitted from the drawing.

The operation of this apparatus will be illustrated by the experiments described in the following examples:

EXAMPLE I This apparatus was used to form a thick fused surface layer on a refractory brick l in a particular experiment in which a fireclay brick was treated. The firebrick was initially preheated to about 800 C. in a muffle furnace and then placed between the sidewalls 2 of the apparatus. A plasma generator of a type using a nozzle as disclosed in our aforementioned copending patent application, of three-sixteenths inches diameter and an axially mounted electrode of l4-inch diameter thoriated tungsten was placed at an angle pointing with the nozzle onto the refractory surface of the firebrick in front of the counter electrode 4. The arrangement of the apparatus was similar to that shown in FIG. I.

The thoriated tungsten electrode was connected to a negative pole and the terminal 6 of the counter electrode to a positive pole of a suitable electrical power supply for the plasma generator. The sidewalls 2 of the apparatus were earthed. A plasma jet was initiated using a gas supply of a mixture of 20cfh argon and l0cfl1 nitrogen. With the are established, a steady current of 200 amperes and an arc voltage of volts was registered.

From the nozzle 7 along and stable arc was produced which travelled for 4 centimeters above the surface of the firebrick and entered the gap between the counter electrode 4 and the upper surface of the firebrick. The counter electrode thus provided a return path for the current in the arc. The heat of the are caused a large area of the refractory surface to be kept at a temperature substantially above its melting point. This molten refractory material lay on the surface of the firebrick in the form of a ridge but when the arc was moved to one side the molten material levelled itself to form a thick flat surface.

When the workpiece was unmelted at the beginning of this operation the plasma jet tended to strike into the space between the counter electrode and the firebrick but as the brick became further heated its emissive properties appeared to increase and the jet then struck more into the brick and a greater proportion of the available heat was then applied to the brick.

The nozzle 7 was arranged to be movable for about 8 cen timeters along the length of the counter electrode so that the whole width of the firebrick could be treated with the plasma effluent in one traverse of the nozzle. At the end of each traverse the firebrick was moved a short distance through the space between the sidewalls 2 so that eventually the whole of the upper surface of the firebrick become treated with the plasma effluent. ln this way the upper surface of the brick was treated at a rate of about 60 square centimeters per minute. Upon completion of the treatment the brick was returned to the muffle furnace at 800 C. and was then allowed to cool at a rate of about 80 C. per hour to room temperature. When the brick was examined it was found that the fusion of the upper surface has caused the production of a glassy layer of l l millimeters in thickness and this had considerably increased the structural strength of the brick as well as making the surface impermeable for most purposes.

EXAMPLE 2 A fireclay brick of the composition:

Si 52.3 percent Cat) 0.2 percent Ti0 0.5 percent Mg0 1.0 percent Fe tl 3.7 percent K 0 0.4 percent A1 0 36/39 percent Na 0 0.2 percent Loss 0.2 percent of 133X98X69 mm. was placed in the apparatus as shown in FIGS. 1 and 2 and had its upper 133 X98 mm. surface treated for 8 minutes by consecutive passes of the plasma gun. The surface which resulted from this treatment was vitreous, flat and very well adhering. The vitreous surface was uniform and approximately 10 mm. thick with variations not greater than :1 mm.

The plasma gun used operated under the following regime:

Voltage: 1 10 V Current: 170A Gases to plasma gun: 8cfh Ar; 35cfh N Additives: None. The surface showed no permeability to alcohol dyes. No spalling, laminations or other cracks were observed. The brick was preheated in a preheat box for 3 hrs. until it reached approximately 700 C. After treatment the brick was allowed to cool down to room temperature in a heat box for approximately 6% hrs.

EXAMPLE 3 Welding of Bricks Two preheated bricks of the same dimensions as in example 2 were placed in the apparatus in an end-to-end position and the abutting region was treated first so as to provide copious quantities of molten material which penetrated into the depth of approximately half the thickness of the brick. Next the whole surface was treated as in example 2, producing a flat continuous virtified surface. A bond formed between the two bricks was found to be'very strong and in every respect it retained the properties of the rest of the treated surface. The operation ofjoining the brick took 10 minutes of plasma time.

The plasma gun operated at:

voltage: V

Current: 185 A Gases to plasma gun: 9cfh Ar; 40cfh N In similar experiments the bond was repeated on the obverse surface, and finally all surfaces of the brick were treated leaving a fully treated brick of twice the original length. No additives were found necessary, although small additions of Ca0 helped to accelerate the fusion.

EXAMPLE 4 A fireclay brick of the same dimension and composition as in example 2, was preheated to a temperature of approximately 700 C., and subjected to plasma treatment. Simultaneously with this treatment a powder containing 1 part of CaO, 2% parts of A1 0 and 6 parts of ZrO (all parts by weight) was fed into the plasma effluents. The powder was dispensed at a uniform rate of approximately 40 g/min. for a period of 4% minutes, in which time the whole area to be treated was fully vitrified. After this treatment the supply of the powder was shut off but the plasma action was continued for another 4 minutes during which time the whole surface was once more (this time rapidly) traversed. The above treatment produced a dense, fused, vitreous surface layer which was approximately 12 mm. thick. The finished brick showed an absence of cracks or laminations and in subsequent tests the surface withstood the attack of furnace slag at a temperature of approximately 1650 C. for a period ofZ hours.

The plasma parameters in the above example were as follows:

Voltage: V

Current: 180 A Gases to plasma gun: locfh Ar; 35cfh N Gases to powder dispenser: 20cfh N F163. 3 and 4 show apparatus that was developed for the treatment of cylindrical refractory bodies such as cylindrical rod sleeves also known as ladle sleeves or rod covers used for the teeming operation in steel making. These sleeves are the subject ofa British Standard Spec. No. 2496: 1954.

The apparatus comprised a fixed main shaft 10 upon which a rod sleeve 11 was mounted between bushings l2 and 13 having male and female supports for the sleeve. The bushings l2 and 13 were rotatable on the shaft 10, the bushing 12 being coupled through a driving disc 14 to a variable speed motor 115. By controlling the motor speed the sleeve 11 carried between the bushings could be caused to rotate about its axis at speeds up to one revolution per minute.

When a sleeve was fitted between the bushings, the bushing 13 was pressed against the end of the sleeve by a spring-loaded fork 16 so that there was little tendency for the sleeve to slip whilst it was being driven round and so that the sleeve would be maintained concentric with the shaft 10.

The motor 15 included an end bearing for the shaft 10 which could be kept cool by the passage of water through the bearing whilst the apparatus was in operation. This use of water cooling is not necessary every time the apparatus is in operation but it can be beneficial when treating very thin sleeves, or sleeves made of good thermal conductors such as magnesite or when the apparatus is being used continuously.

A range of bushings was made for the apparatus so that with suitable spacers the treatment ofa variety of rod sleeves of different sizes and diameters could be undertaken.

Positioned around the sleeve, electrical radiant heating elements 17 were located within a cylindrical reflector 18. The elements served to keep the sleeve as a whole at a suitably high temperature whilst it was undergoing treatment in the apparatus.

A water cooled counter electrode 19 was mounted close to the sleeve 11, and pointing roughly into the space between electrode and sleeve was a plasma jet nozzle 20. The plasma jet nozzle was connected to an electrical power supply 20a and it was also provided with suitable gases from a gas supply 20b. The plasma jet nozzle was coupled to a cable 21 movable by means of a drive motor 22 so that the nozzle could be driven along the axis of the sleeve. The nozzle was pivoted on a bush which could slide along a shaft 23 so that the nozzle might be adjusted in height and angle with respect to the sleeve 11. The nozzle was also supported by a gliding rail 24 which aided the smooth traversing action of the nozzle and the position of this rail was also adjustable. Movement of the nozzle was further facilitated by the provision of an adjustable counterweight 25. The provision of these adjustments was found to promote conditions in which when the apparatus is in operation the melted but very viscous surface of the sleeve did not flow out but tended to set as a smooth finish on the periphery of the sleeve.

Close to the point where a plasma jet emerged from the nozzle 20 was a dispenser 26 for the injection of powders into the plasma stream. 1 i

The end of the main shaft remote from the motor was supported in a removable rest 27 which included a thermocouple for measuring the approximate temperature of the shaft during operation of the apparatus.

The operation of this apparatus will be illustrated by the experiments described in the following examples:

EXAMPLE 5 A fireclay sleeve rod of 12 inches height 4 inches outside diameter and 1% inches inside diameter was preheated to a temperature of 650 C. and treated in the apparatus shown in FIGS. 3 and 4. The original composition of the sleeve was as in example 2. Additives were introduced into the plasma stream comprising 1 part by weight of calcium oxide (CaOZ, 4 parts of alumina (M 0 and 14 parts of zirconia (2100:) all carried in nitrogen, for a period of 9 minutes, in which time one complete revolution of the sleeve was accomplished. The plasma treatment was prolonged for another 9 minutes during which another complete revolution of the sleeve was carried out. The sleeve was removed from the apparatus and allowed to cool for 8 hours to room temperature in an insulated heat box.

Plasma parameters were as follows:

Voltage: 130 V Current: 250 A Gases to plasma gun: 30cfh Ar; 40cfl1 N Gases to powder dispenser: cfh N EXAMPLE 6 A fireclay rod sleeve of the same type as in example 5 was preheated to approximately 700 C. and treated as in the previous example. The additives introduced into the plasma stream comprised: 1 part by weight of CaO, 2 parts of powdered aluminum and 16 parts of zircon powder (ZrO SiO consisting of approximately 65 percent ZrO and 34 percent SiO for a period of 10 minutes during which time one complete revolution of the sleeve was accomplished. The powders were introduced at 40g/min. A further one revolution of the sleeve was carried out under plasma effluents in 8 minutes and the sleeve was allowed to cool in the preheated, insulated heat box for 8 hours to room temperature.

The plasma parameters were approximately as in example 5.

EXAMPLE 7 In this example a different technique was tried, viz. that of painting the surface of the sleeve with the composition of the parts of M gO' and 20 parts of monazite sand (containing 7 percent thoria) was suspended in a small quantity of water. Linseed oil was added to the suspension in an amount about onethird the volume of water. The resulting paste was applied liberally to the outer surface of the sleeve, using 500 g. per sleeve.

The sleeve was inserted in to the heat box and preheated to approximately 650 C. in three hours and then subjected to 15 min. plasma treatment, using the same plasma parameters as in example 5.

The sleeve was cooled to ambient temperature in four hours and showed good resistance under basic conditions.

The reason for the success of this method of using a counter electrode in a transferred arc plasma generator when treating a normally nonconductive material like the ceramic refractory is believed to be due to the emissive properties of the refractory. Atthe temperatures to which the ceramic is heated by the plasma jet the ceramic appears to become appreciably electrically conductive by virtue of its ionized vapor and liquid phases. A large molten pool of the refractory ceramic can be thus formed. This large pool may be used advantageously when it is required to join two refractory bodies. These may be placed side by side in the apparatus of the invention and upon the creation of the large pool of molten ceramic along their common edge the melted ceramic will penetrate the gap between the two bodies which when solidified will give a strong bond across the gap. 5

A further advantage of this method of treating a refractory brick is the possibility of giving the fused surface layer of the brick different properties to the material of the remainder of the brick. For instance, a refractory brick which is to be exposed in service to the action of molten steel or slag may be given a surface layer which includes an additive such as zirconium oxide. This additive may in introduced into the molten pool of ceramic by a technique such as injection into the stream comprise gases comprising the plasma jet or by spraying a suitable zirconium compound into the space between the counterelectrode 4 and the refractory brick. This method can provide a brick molten a zirconium rich surface which may be made at a very much lower cost than a brick which is wholly of zirconium rich material. in this way an ordinary refractory brick may be upgraded by being given a surface coating which will endow the brick with greatly enhanced refractory properties or any other properties according to the application proposed for the brick.

It has been stated that for a fireclay brick, the most important properties required for service are that it shall have high density and low permeability. The surface treatment according to the invention can enable a brick to have a surface including a dense fused layer with these qualities present in an enhanced degree. A sleeve may also be given such a smooth thick and dense layer without use of any additives other than mere exposure to the plasma stream. A sleeve however must have properties providing a compromise between the high porosity necessary to keep cool a steel rod which the sleeve is required to protect when in service and a low porosity to resist attack by molten slag in contact with the sleeve. It is possible however to modify the properties of the surface layer in a useful way by the introduction of selected additives. This may be effected for example by additions made in one of the following ways:

I. Directly into the plasma jet emerging from the jet nozzle or by feeding the additive into a constricting passage of an arc chamber of the nozzle.

2. By introducing one or more constituents into the plasma are which will react in the presence of plasma so that the products reaching the surface are substantially changed. For instance, by feeding fine aluminum powder into nitrogen plasma, a fair proportion of aluminum nitride would result.

3. By impregnating the surface of the refractory with substances which will bond themselves into the matrix of the refractory and/or react with it or with plasma gases during the subsequent surface treatment.

Modification of the refractory surface by the addition of zirconia Zro,, zircon ZrSiO, alumina and thoria Th0 has already been described in the foregoing paragraphs but the use of other additives including carbon, aluminum nitride AlN titanium nitride and diboride and forsterites have been proposed. Mixtures such as one of alumina with forsterite, possibly including titanium and aluminum nitrides may also be used.

The foregoing descriptions of embodiments of the invention have been given by way of example only and a number of modifications may be made without departing from the scope of the invention. For instance, instead of the counter electrode being of copper sheet with cooling pipes it might alternatively be a closed box of thin copper sheet with water circulating within the walls of the box. The use of the counter electrode of the invention is also not restricted to transferred arc operations on ceramic materials, many other ordinarily nonconductive materials such as types of natural minerals will also have their electrical conductivity sufficiently enhanced at elevated temperatures and will be suitable for treatment with plasmajet apparatus using the counter electrode.

We claim:

I. A method of sealing a surface of a nonconductive refractory ceramic body. comprising the steps of:

positioning a counter electrode for a plasma gun alongside a selected portion ofthe ceramic surface;

producing a stream of plasma effluent from a plasma gun operable in a transferred arc mode;

directing said plasma stream obliquely at said surface and towards said counter electrode, said stream directed between said counter electrode and said ceramic surface; fusing a portion of said ceramic surface;

moving said plasma stream from end to end along said counter electrode and said surface so as to enlarge said fused portion; and

allowing said fused portion of surface to solidify to form a ceramic glaze in situ on the ceramic body to thus seal said surface.

2. A method as claimed in claim 1 comprising the step of coating the ceramic surface with an additive material before the plasma treatment such additive material being chosen so as to improve a property of the ceramic glaze eventually to be produced.

3. A method as claimed in claim 1 comprising the step of spraying an additive material on to the ceramic surface during the sealing operation.

4. A method as claimed in claim 1 comprising the step of preheating the refractory ceramic body in a furnace before the sealing operation.

5. A method as claimed in claim 1 comprising the step of directing radiant heat on to the surface of the ceramic body during the sealing operation.

6. A method as claimed in claim 1 comprising the step of rotating the ceramic body during the sealing operation.

7. A method as claimed in claim 1 comprising the step of liquid-cooling the counter electrode during operation of the plasma gun. 

2. A method as claimed in claim 1 comprising the step of coating the cerAmic surface with an additive material before the plasma treatment such additive material being chosen so as to improve a property of the ceramic glaze eventually to be produced.
 3. A method as claimed in claim 1 comprising the step of spraying an additive material on to the ceramic surface during the sealing operation.
 4. A method as claimed in claim 1 comprising the step of preheating the refractory ceramic body in a furnace before the sealing operation.
 5. A method as claimed in claim 1 comprising the step of directing radiant heat on to the surface of the ceramic body during the sealing operation.
 6. A method as claimed in claim 1 comprising the step of rotating the ceramic body during the sealing operation.
 7. A method as claimed in claim 1 comprising the step of liquid-cooling the counter electrode during operation of the plasma gun. 